Isolating selection mechanisms in beach seines

The selection mechanisms of beach seines were investigated during two separate experiments in New South Wales (NSW), Australia to determine the effects on catches associated with changing mesh size in the (i) posterior wings and (ii) bunt. Irrespective of the seine configuration, in all deployments fewer than 8% of fish were meshed in the anterior wings (combined length of 375 m and a mesh size of 80 mm), while between 11 and 40% were meshed in the posterior wings (combined length of 60 m) and between 64 and 87% were retained in the bunt (total length of 12.5 m). In experiment 1, increasing the mesh size from 57 (conventional) to 80 mm in the posterior wings had significant (p < 0.05) effects on the size distributions of three key species in the posterior wings and two species in the total seine. But due to temporal variability in catch volume, the effects on catch weights and numbers were not significant. In experiment 2, maintaining mesh size in the posterior wings at 63 mm while increasing mesh size in the bunt (from 33 to 63 and 80 mm) significantly (p < 0.05) reduced the catches of undersized individuals of three key species in this latter section. However, variability among catches in the posterior wings meant that significant effects were maintained for only two of these species for the entire seine. Such broad effects of the posterior section of the gear were attributed to the operational characteristics and species-specific differences in selection mechanisms. Owing to the orientation of meshes in the posterior wings, size selection for many species was considerably more defined than in the bunt. But because most fish were eventually directed into the bunt, this area demonstrated a greater potential for improving selection. We conclude that increasing the smallest minimum mesh size (30 mm) used in NSW's beach seines by almost 100% to correlate with the maximum girth of one of the smallest legal-sized species, would be an appropriate first step for reducing unwanted fishing mortality, but that further work is required to investigate the utility of other more novel modifications throughout the gear, and especially in the bunt.